Physiology of Nerves and Nerve-cells . 505 



to convey a stimulus to a muscle. Helmholtz found the speed of the 

 propagation of nervous stimulation to be about 78 feet per second. It 

 is not quite constant, being greater in higher and less in lower tempera- 

 tures. It is possible to agitate the human nerves through the skin, es- 

 pecially when they are situated i:ear the surface, since the skin is a fair 

 conductor. Hclmholtz' experiments on the live human nerve showed a 

 speed of about 97 feet per second, the higher rate accounted for by the 

 higher temperature of the warm blooded over the cold blooded animal. 

 This is the average speed of the propagation of the stimulus. But it is 

 shown that the progress of the stimulus at first rapid, is gradually re- 

 tarded, going slower toward the last ; a fact due to the resistance of the 

 conductor. If the main stem of a nerve is irritated by electric shocks, 

 all the fibres composing it, and into which it branches, partake of the 

 shock, but the insulation of the fibres is such that an irritation, however, 

 made in a separate fibre, is not communicated to the other, and will ex- 

 cite only its own tract of muscle. The value of the insulation of a con- 

 ductor depends largely upon the force of the current. Thus a telegraph 

 circuit with its heavy out-door wire, its instruments and their light in- 

 sulated wire connections, is sufficiently insulated to be operated by a 

 light galvanic current without loss. But the approach of electric clouds 

 may so charge the circuit as to burn out the instruments and make it 

 unsafe for the operators. So we may understand that a sufficiently in- 

 sulated conductor for a nervous current may be constructed of materials 

 which are not absolutely non-conductors of electricity. The current 

 takes the direction of least resistance which is along the nerve rather 

 than across it. The resistance to galvanic conduction across a fibre of 

 living nerve, is five times as great as the longitudinal resistance, and 

 across a muscle fibre it is seven times as great as it is lengthwise. 

 ( Landois and Stirling.) 



Whatever lack of perfection there may be in the insulation of the 

 nerves, and particularly their ultimate fibrillae, it is compensated by the 

 character of the terminal organs themselves, whether muscle fibres, glands, 

 or nerve or cerebral cells. The character of each of these organs is such 

 that it is susceptible to onfy a certain quality or pitch of stimulation, 

 and unless assailed by that particular sort of stimulation will not re- 

 spond in normal action. This I believe to be especially true of the sen- 

 sory receptive organs in the brain, which there is reason to believe are 

 deaf and blind to every tone not exactly pitched to correspond with their 

 fundamental, or the tone by which their differentiation was effected, as it 

 is obviously true of the external sensory organs. 



Nerves are variable in their facility of excitation one nerve differ- 

 ing from another in this respect, one part of the same nerve dinVriiuc 

 from another part, and the same nerve (littering from itself at dill 



